<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .cl1{
           
            width: 400px;
            height: 200px;
            border: 1px solid black;
            margin: 50px auto;
        }
        .cl2{
            position: relative;
            left: 50%;
            width: 800px;
            height: 200px;
            margin-left: -400px;
            background-color:paleturquoise;
        }
    </style>
</head>
<body>
    <div class="cl1">
    <div class="cl2"></div>
    </div>
</body>
</html>